fix(inbox): restore Scout in the source filter dropdown

[andrewm4894]

Problem

Inbox 2.0 (#2547) rebuilt the source filter from a new INBOX_SOURCE_OPTIONS list and dropped the signals_scout entry added in #2505. Scout-sourced reports could no longer be filtered: the dropdown had no Scout row, and a persisted Scout filter rendered its chip with the raw signals_scout value because the label lookup falls back to the raw string when no option matches.

Changes

• Add Scout (signals_scout, compass icon) back to INBOX_SOURCE_OPTIONS in packages/ui/src/features/inbox/filterOptions.tsx, so the source dropdown lists Scout again and the filter chip reads "Scout".
• Reword the inbox CLAUDE.md rule that said "Do not add Scout UI until a corresponding backend exists in this repo" — it reads as if Scout shouldn't appear anywhere, which is likely how the filter entry got dropped. It now states Scout is a real Cloud source product that must stay covered in filter options, source meta, and signal cards, and scopes the prohibition to Scout management UI (fleet configuration, run history).

Scout coverage that survived 2.0 and is unchanged here: SOURCE_PRODUCT_META badges and the scout-name source line on SignalCard (#2538).

How did you test this?

• pnpm --filter @posthog/ui typecheck passes.
• pnpm --filter @posthog/ui test — 690 tests pass.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

[greptile-apps]

_{Reviews (1): Last reviewed commit: "fix(inbox): restore Scout in the source ..." | Re-trigger Greptile}

[github-actions]

Trivial additive UI change — adds a Scout entry to the source filter dropdown and updates the local CLAUDE.md guidance to match. No logic changes, no data model impact, no security concerns.